1. Field of the Invention
The present invention relates to a magnesium-based alloy with superior fluidity and hot-tearing resistance, and a manufacturing method thereof.
2. Description of the Prior Art
Generally, since magnesium alloy or magnesium is the lightest element among commercially available metals and is excellent in specific strength and specific stiffness, it is being expected as a light structure material.
Magnesium with a specific gravity of 1.7 is not only the lightest element among commercially available metals, but its specific strength and specific stiffness are also superior to those of iron and aluminum. In addition, excellent mechanical properties can be obtained when manufacturing magnesium products by a die casting process. Therefore, magnesium is currently being applied to various fields, such as portable electronic components, aircrafts and sporting goods, etc., with mainly focusing on the field of automobile components. When magnesium alloys are applied to the automobile components, 30% of a weight reduction can be achieved.
Typical magnesium alloys among the currently available commercial magnesium alloys for die casting applications are magnesium (Mg)-aluminum (Al) based alloys such as AZ91D, AM50 and AM60. Properties required for magnesium alloy are corrosion resistance and oxidation resistance as well as castability suitable for die casting. Moreover, when considering competitiveness against steel and aluminum, development of magnesium alloys excluding high-priced additive elements is required in terms of cost.
Magnesium alloys which have been developed based on the above requirements are disadvantageous in cost in the case where an addition ratio of a rare earth element (RE) is increased. On the other hand, when adding alkaline earth metals (e.g., calcium (Ca) and strontium (Sr)) into magnesium alloys, there is a problem that the magnesium alloys have poor castability such as decrease in melt fluidity, hot tear cracks, and die soldering. The price of calcium is about $200/kg, causing the manufacturing cost of magnesium alloy to be increased.
Furthermore, in the case where alkaline earth metal (Ca or Sr) is directly added into magnesium or magnesium alloy, a portion of the alkaline earth metal is dissolved in the magnesium alloy. Therefore, in order to improve physical properties of the magnesium alloy, addition of more than a certain fraction of the alkaline earth metal is required. For example, Ca should be added in an amount of 1.34 wt % or more (0.8 wt % or more in a non-equilibrium state) in order to be undissolved in a magnesium matrix and to form an intermetallic compound with magnesium or other alloying elements, wherein the intermetallic compound affects physical properties of an alloy.